Load color tag presets from an editable presets.json file

The presets move from app.js to presets.json, built into the binary and
served by /api/presets. PRESETS_FILE (default /data/presets.json) replaces
the list when present; it is read again each time Settings opens and the
built-in list is used if it is invalid. docs/presets.md (EN/FR) explains
each preset and the file format.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

presets_test.go

@@ -0,0 +1,46 @@
+package main
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestBuiltinPresets(t *testing.T) {
+	groups, err := parsePresets(builtinPresets)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(groups) == 0 {
+		t.Fatal("no built-in presets")
+	}
+}
+
+func TestLoadPresetsFile(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "presets.json")
+
+	if res := loadPresets(path); res.Source != "builtin" || res.Error != "" {
+		t.Fatalf("missing file: got %q, error %q", res.Source, res.Error)
+	}
+
+	os.WriteFile(path, []byte(`[{"group":"Mine","presets":[{"id":"a","name":{"en":"A","fr":"A fr"},
+		"tags":[{"label":"x","pattern":"foo|bar","color":"#112233"},{"pattern":"a.b","color":"#445566","regex":false}]}]}]`), 0o644)
+	res := loadPresets(path)
+	if res.Source != "file" || res.Error != "" || res.Groups[0].Presets[0].Name["fr"] != "A fr" || res.Groups[0].Group["en"] != "Mine" {
+		t.Fatalf("valid file: %+v", res)
+	}
+
+	for _, bad := range []string{
+		`not json`,
+		`[{"group":"G","presets":[{"id":"a","name":"A","tags":[{"pattern":"(","color":"#112233"}]}]}]`,
+		`[{"group":"G","presets":[{"id":"a","name":"A","tags":[{"pattern":"x","color":"red"}]}]}]`,
+		`[{"group":"G","presets":[{"id":"a","name":"A","tags":[]}]}]`,
+		`[{"group":"G","presets":[{"id":"a","name":"A","tags":[{"pattern":"x","color":"#112233"}]},{"id":"a","name":"B","tags":[{"pattern":"y","color":"#112233"}]}]}]`,
+	} {
+		os.WriteFile(path, []byte(bad), 0o644)
+		if res := loadPresets(path); res.Source != "builtin" || res.Error == "" || len(res.Groups) == 0 {
+			t.Errorf("%s: got %q, error %q", bad, res.Source, res.Error)
+		}
+	}
+}